The present invention relates to an optical disk apparatus for optically reading information from an optical storage medium. More particularly the present invention relates to an optical head for reading signals from optical disks having different substrate thicknesses by using light sources having different optical wavelengths, and an objective lens for use in such optical head.
Optical disks have recently been making remarkable advances as large capacity-removable-information storage media. Accordingly, writing-reading methods, storage densities and disk sizes have taken on great diversity. Thus, it is becoming very difficult to ensure compatibility among the different systems. Among other things, CDs (Compact Disc(s)) are presently most popular, and CD-Rs (Compact Disk-Recordable(s)) which are recordable CDs having reading compatibility with CDs are becoming equally as popular. It is desired that development of new optical disks meet the important demand for compatibility with such CDs and CD-Rs.
Recently DVDs (Digital Video Disk(s)), the next generation of high density ROM following CDs and CD-Rs, have been introduced to the market. To increase the storage density of a DVD, the numerical aperture (NA) of an objective lens is increased from 0.45 for the conventional CDs up to 0.6. Letting .lambda. be the wavelength of a laser source to be used, the size of a focused spot on an optical disk is proportional to .lambda./NA, so that as the wavelength is made shorter and the NA larger, the size of the beam spot can be made smaller. If the size of the beam spot is small, it is possible to read high-density information pits with good quality, so that the storage density of the optical disk can be increased.
In light of the above, the wavelength of a semiconductor laser used for DVDs is 650 nm instead of 780 nm for CDs. However, since an increase in the NA sharply increases coma which occurs when a disk tilts, and rather degrades the beam spot, it is impossible to excessively increase the NA. For this reason, DVDs have a substrate thickness of 0.6 mm thinner than 1.2 mm of CDs so that the NA can be increased and the accompanying coma due to a disk tilt can be reduced. However, since the substrate thickness of DVDs differs from that of CDs, if a CD is read with a DVD-dedicated objective lens, a spherical aberration will occur and the beam spot will defocus. This occurs because objective lenses for optical disks are respectively intended for particular substrate thicknesses and are beforehand designed to have spherical aberrations which compensate for the particular substrate thicknesses.
Conventional apparatus for solving the above problem is described in, for example, Optical Review, Vol. 1, No. 1 (1994) pp. 27-29). In this conventional apparatus, a hologram is formed on the surface of an objective lens for 0.6 mm disks, and a CD is read with diffracted light, while a DVD is read with transmitted light. The pattern of the hologram is beforehand designed so as to compensate for spherical aberration which occurs during CD-read. However, in this conventional apparatus, since the hologram is used, a beam spot for DVDs is produced even during a CD-read operation, whereas a beam spot for CDs is produced even during a DVD-read operation. In addition, a beam reflected from a disk is again diffracted. This leads to the disadvantage of unavoidable loss of light power.
A second conventional apparatus is described in Mitsubishi Electric Co. Ltd. News Release, No. 9507 (Jun. 21, 1995). In the second conventional apparatus, both an objective lens for 0.6 mm disks and an objective lens for 1.2 mm disks are provided on an optical head, and the two lenses are switched when needed by a movable actuator. However, in this example, since the two lenses are switched when needed, there are problems such as an increase in cost due to the use of two lenses, the reproducibility of the positions of the lenses, and the degradation of response characteristics due to a large and heavy actuator.
A third conventional apparatus is described in Nikkei Electronics, Jan. 29, 1996 (No. 654), pp. 15-16. In the third conventional apparatus an aperture limitation using a liquid crystal is provided, and during a CD-read operation, the NA is reduced to 0.35 so as to reduce aberration. Since a semiconductor laser of wavelength 635 nm is used for both CDs and DVDs, the NA for CDs can be reduced to some extent. There is, however, a disadvantage in that this method cannot be used for reading CD-Rs whose reflectance becomes quite low for a beam of wavelength shorter than 780 nm.
A fourth conventional apparatus is described in Japanese Patent Application No. 342203/1995. The fourth conventional apparatus provides an objective lens in which the inner and outer regions are given different optimized substrate thicknesses, so as to realize compatibility between both DVDs and CDs at a wavelength of 650 nm. However, if a CD is to be read at a wavelength of 780 nm, this boundary NA needs to be made at least 0.45 or more, but this case leads to the disadvantage that the aberration for DVD-read becomes extremely large.